1. Field of the Invention
The present invention relates to the respiratory care of a patient and, more particularly, to a ventilator that monitors the pressure and flow rate of the breathing gas supplied to and exhaled from the patient, advises the operating clinician regarding the appropriate quality and quantity of ventilation support corresponding to the patient""s needs, and, alternatively, controls the pressure and/or flow rate of the breathing gas supplied by the ventilator to provide the appropriate quality and quantity of ventilation support to the patient to maintain a desired work of breathing level in the patient.
2. Prior Art
Mechanical ventilatory support is widely accepted as an effective form of therapy and means for treating patients with respiratory failure. Ventilation is the process of delivering oxygen to and washing carbon dioxide from the alveoli in the lungs. When receiving ventilatory support, the patient becomes part of a complex interactive system which is expected to provide adequate ventilation and promote gas exchange to aid in the stabilization and recovery of the patient. Clinical treatment of a ventilated patient often calls for monitoring a patient""s breathing to detect an interruption or an irregularity in the breathing pattern, for triggering a ventilator to initiate assisted breathing, and for interrupting the assisted breathing periodically to wean the patient off of the assisted breathing regime, thereby restoring the patient""s ability to breath independently.
In those instances where a patient requires mechanical ventilation due to respiratory failure, a wide variety of mechanical ventilators are available. Most modern ventilators allow the clinician to select and use several modes of inhalation either individually or in combination. These modes can be defined in three broad categories: spontaneous, assisted or controlled. During spontaneous ventilation without other modes of ventilation, the patient breathes at his own pace, but other interventions may affect other parameters of ventilation including the tidal volume and the baseline pressure, above ambient, within the system. In assisted ventilation, the patient initiates the inhalation by lowering the baseline pressure by varying degrees, and then the ventilator xe2x80x9cassistsxe2x80x9d the patient by completing the breath by the application of positive pressure. During controlled ventilation, the patient is unable to breathe spontaneously or initiate a breath, and is therefore dependent on the ventilator for every breath. During spontaneous or assisted ventilation, the patient is required to xe2x80x9cworkxe2x80x9d (to varying degrees) by using the respiratory muscles in order to breath.
The work of breathing (the work to initiate and sustain a breath) performed by a patient to inhale while intubated and attached to the ventilator may be divided into two major components: physiologic work of breathing (the work of breathing of the patient) and breathing apparatus imposed resistive work of breathing. The work of breathing can be measured and quantified in Joules/L of ventilation. In the past, techniques have been devised to supply ventilatory therapy to patients for the purpose of improving patient efforts to breath by decreasing the work of breathing to sustain the breath. Still other techniques have been developed that aid in the reduction of the patient""s inspiratory work required to trigger a ventilator system xe2x80x9cONxe2x80x9d to assist the patient""s breathing. It is desirable to reduce the effort expended by the patient in each of these phases, since a high work of breathing load can cause further damage to a weakened patient or be beyond the capacity or capability of small or disabled patients.
The early generation of mechanical ventilators, prior to the mid-960s, were designed to support alveolar ventilation and to provide supplemental oxygen for those patients who were unable to breathe due to neuromuscular impairment. Since that time, mechanical ventilators have become more sophisticated and complicated in response to increasing understanding of lung pathophysiology. Larger tidal volumes, an occasional xe2x80x9csigh breath,xe2x80x9d and a low level of positive end-expiratory pressure (PEEP) were introduced to overcome the gradual decrease in functional residual capacity (FRC) that occurs during positive-pressure ventilation (PPV) with lower tidal volumes and no PEEP. Because a decreased functional residual capacity is the primary pulmonary defect during acute lung injury, continuous positive pressure (CPAP) and PEEP became the primary modes of ventilatory support during acute lung injury.
In an effort to improve a patient""s tolerance of mechanical ventilation, assisted or patient-triggered ventilation modes were developed. Partial PPV support, where mechanical support supplements spontaneous ventilation, became possible for adults outside the operating room when intermittent mandatory ventilation (IMV) became available in the 1970s. Varieties of xe2x80x9calternativexe2x80x9d ventilation modes addressing the needs of severely impaired patients continue to be developed.
The second generation of ventilators was characterized by better electronics but, unfortunately, due to attempts to replace the continuous high gas flow IMV system with imperfect demand flow valves, failed to deliver high flow rates of gas in response to the patient""s inspiratory effort. This apparent advance forced patient""s to perform excessive imposed work and thus, total work in order to overcome ventilator, circuit, and demand flow valve resistance and inertia. In recent years, microprocessors have been introduced into modern ventilators. Microprocessor ventilators are typically equipped with sensors that monitor breath-by-breath flow, pressure, volume, and derive mechanical respiratory parameters. Their ability to sense and transduce xe2x80x9caccurately,xe2x80x9d combined with computer technology, makes the interaction between clinician, patient, and ventilator more sophisticated than ever. The prior art microprocessor controlled ventilators suffered from compromised accuracy due to the placement of the sensors required to transduce the data signals. Consequently, complicated algorithms were developed so that the ventilators could xe2x80x9capproximatexe2x80x9d what was actually occurring within the patient""s lungs on a breath by breath basis.
Unfortunately, as ventilators become more complicated and offer more options, so the number of potentially dangerous clinical decisions increases. The physicians, nurses, and respiratory therapists that care for the critically ill are faced with expensive, complicated machines with few clear guidelines for their effective use. The setting, monitoring, and interpretation of some ventilatory parameters have become more speculative and empirical, leading to potentially hazardous misuse of these new ventilator modalities. For example, the physician taking care of the patient may decide to increase the positive pressure level based on the ventilator displayed high spontaneous breathing frequency and low exhaled tidal volume. This approach, unfortunately, threatens the patient with the provision of inappropriate levels of pressure support.
Ideally, ventilatory support should be tailored to each patient""s existing pathophysiology rather than employing a single technique for all patients with ventilatory failure. Thus, current ventilatory support ranges from controlled mechanical ventilation to total spontaneous ventilation with CPAP for support of oxygenation and the elastic work of breathing and restoration of lung volume. Partial ventilation support bridges the gap for patients who are able to provide some ventilation effort but who cannot entirely support their own alveolar ventilation. The decision-making process regarding the quality and quantity of ventilatory support is further complicated by the increasing knowledge of the effect of mechanical ventilation on other organ systems.
The overall performance of the assisted ventilatory system is determined by both physiological and mechanical factors. The physiological determinants, which include the nature of the pulmonary disease, the ventilatory efforts of the patient, and many other anatomical and physiological variables, changes with time and are difficult to diagnois. Moreover, the physician historically had relatively little control over these determinants. Mechanical input to the system, on the other hand, is to a large extent controlled and can be reasonably well characterized by examining the parameters of ventilator flow volume and/or pressure. Optimal ventilatory assistance requires both appropriately minimizing physiologic workloads to a tolerable level and decreasing imposed resistive workloads to zero. Doing both should insure that the patient is neither overstressed nor oversupported. Insufficient ventilatory support places unnecessary demands upon the patient""s already compromised respiratory system, thereby inducing or increasing respiratory muscle fatigue. Excessive ventilatory support places the patient at risk for pulmonary-barotrauma, respiratory muscle deconditioning, and other complications of mechanical ventilation.
Unfortunately, none of the techniques devised to supply ventilatory support for the purpose of improving patient efforts to breath, by automatically decreasing imposed work of breathing to zero and appropriately decreasing physiologic work once a ventilator system has been triggered by a patient""s inspiratory effort, provides the clinician with advice in the increasingly complicated decision-making process regarding the quality and quantity of ventilatory support. As noted above, it is desirable to reduce the effort expended by the patient to avoid unnecessary medical complications of the required respiratory support.
From the above, it is clear that it would be desirable to have a medical ventilator that reduces the patient""s work of breathing toward an optimum level by alerting the clinician of the ventilator""s failure to supply the appropriate quality and quantity of ventilatory support and by providing advise to the clinician regarding the appropriate quality and quantity of ventilatory support that is tailored to the patient""s pathophysiology. Further, it would be desirable to have such a ventilator that, in addition to alerting and advising the clinician, also automatically changes the quality and quantity of ventilatory support that is required to support a patient""s current pathophysiology. Such a ventilatory system is unavailable in current systems.
An excessively high expenditure of energy (work of breathing) by the patient, early in the inspiratory phase of ventilation, can be detrimental to the patient. Patients may fatigue under these workloads, leading to further respiratory distress and/or failure. The required energy expenditure can also create difficulties in weaning the patient from the ventilator, leading to patients who become ventilator dependent. Thus, reducing the energy expenditure to an appropriate level while breathing spontaneously on a mechanical ventilator is advantageous for the patient. Pressure support ventilation is a commonly used mode of ventilatory support employed to decrease a patient""s work of breathing or effort to inhale to appropriate levels. The principle object of the present invention is to provide an open-loop method and corresponding apparatus for determining and advising the operating clinician of the pressure support level of ventilation support provided to the patient that will maintain the patient work of breathing within a desired work of breathing range selected by the operating clinician.
A further object of the present invention is to provide a closed-loop method and corresponding apparatus for continually and automatically adjusting the selected pressure support level of ventilation support provided to the patient to maintain the patient work of breathing within the selected predetermined work of breathing range for any selected period of time.
Conventional approaches of applying pressure support ventilation are based on either: a) assessing a patient""s breathing pattern, or b) directly measuring the work of breathing of the patient. If the breathing pattern is inappropriate (i.e., the breathing rate is too fast), then the work of breathing is too high or too low and the pressure support ventilation level should be adjusted until the breathing pattern or work of breathing is are physiologically appropriate. However, it has been determined that the assessment of the breathing pattern is an inaccurate assessment of the work of breathing of the patient. Additionally, the direct measurement of the work of breathing of the patient may be difficult and requires special and expensive equipment not available to most physicians. Therefore, it is desirable to have an accurate means for determining the work of breathing of the patient for the application of pressure support ventilation.
Another objective of the present invention results from the discovery that there is a strong correlation between the average respiratory muscle pressure of a patient and the patient work of breathing. Therefore, a further objective of the invention is to provide a simple and easy method and apparatus for controlling a medical ventilator based upon readily measured exhalation gas parameters and the readily determined average respiratory muscle pressure of the patient.
It is yet another objective of the invention to provide a method and apparatus for accurately predicting the patient work of breathing based upon maximizing the correlation between the readily determined average respiratory muscle pressure of the patient and the work of breathing of the patient.
It is another objective of the invention to provide a method and apparatus for nullifying the work of breathing imposed by breathing apparatus by continually modulating the pressure and/or flow rate of the breathing gas supplied by the ventilator to maintain the pressure of the breathing gas near the distal end of a breathing attachment, such as an endotracheal tube, at a constant, predetermined, baseline pressure throughout an inhalation effort of the patient.
The present invention is directed to an open- or closed-loop method and corresponding apparatus for providing breathing gas to a patient such that the patient exerts a desired work of breathing during pressure support ventilation, and such that the work of breathing of the patient is monitored and the pressure and/or flow rate of the breathing gas provided to the patient is controlled throughout the inspiratory phase to provide a pressure support ventilation level that provides the desired work of breathing in the patient.
Briefly, the present invention is directed to a medical ventilator for supplying a breathing gas for use in a medical procedure, such as pressure support ventilation, at a selected pressure support ventilation level. The breathing gas being received into the medical ventilator is from a gas source of one or more breathing gases and the gas exiting the ventilator is in flow communication with a functionally open ventilator conduit. The ventilator conduit has a patient breathing attachment, such as an endotracheal tube, in fluid communication with the lungs of the patient. A pressure sensor is disposed in the ventilator conduit that senses the pressure of the breathing gas within the ventilator conduit and a flow rate sensor is disposed in the ventilator conduit to detect the flow rate of the breathing gas within the ventilator conduit. A monitoring means, such as a microprocessor, is connected to the pressure sensor and to the flow rate sensor to monitor the patient work of breathing, to detect when the patient work of breathing is not within a predetermined work of breathing range, and to generate a response signal thereof The ventilator apparatus further has an alarm means responsive to the response signal to generate an alarm suitable for alerting an operator that the patient work of breathing is not within the predetermined work of breathing range.
The medical ventilator also has a gas delivery means that is in fluid/flow communication with the gas source for receiving the breathing gas from the gas source. The gas delivery means regulates the pressure and/or flow rate of the breathing gas to supply the breathing gas to the patient at the selected pressure support ventilation level. Further, the medical ventilator has a regulating means operatively coupled to the gas delivery means and the monitoring means for pressure and/or flow rate controlling the breathing gas supplied to the patient so that the breathing gas may be delivered to the patient at a selected pressure support ventilation level. The gas delivery means comprises a pneumatic system, having at least one actuator, responsive to the monitoring means via the regulating means, for controlling the pressure and/or the flow rate of the breathing gas so that the selected pressure support ventilation level is provided to the patient. The regulating means, responsive to the response signal from the monitoring means that indicates that the monitored work of breathing of the patient is not within the desired work of breathing range, may adjust the selected pressure support ventilation level of the breathing gas provided to the patient by the ventilator until a pressure support ventilation level is reached such that the patient work of breathing is within the predetermined work of breathing range.
To advise the clinician of the medical parameters being monitored or determined, the medical ventilator may also have a signal output means, such as a monitor, for displaying electronic output signals for concurrent review by a clinician. The electronic output signals may include at least one or more of: the stored signals, the predetermined work of breathing range, the determined work of breathing of the patient, the average respiratory muscle pressure of the patient, the selected pressure support level of the ventilator, and the target pressure support ventilation level. The display of the target pressure support level of the ventilator advises the clinician operating the medical ventilator with the ventilator pressure support level that will provide breathing support to the patient during pressure support ventilation that will maintain the patient""s work of breathing within the desired work of breathing range (i.e., the target pressure support ventilation level).
Moreover, the present invention relates to a method of providing, for any selected period of time, pressure support ventilation to a patient supplied with a breathing gas from a medical ventilator, the gas being pressure and/or flow rate controlled by the ventilator, comprising the steps of delivering the breathing gas from the ventilator to the patient via a ventilator conduit at a selectable pressure support ventilation level; sensing the pressure of the breathing gas within the ventilator conduit; measuring the flow rate of the breathing gas within the ventilator conduit; monitoring the work of breathing of the patient from the sensed pressure and measured flow rate of the breathing gas; and alarming the operating clinician when it is determined that the patient work of breathing is not within the predetermined work of breathing range. The method of the present invention may also comprise the steps of controlling the pressure support level of the breathing gas when it is determined that the patient work of breathing is not within the predetermined work of breathing range and displaying the selected pressure support ventilation level to the clinician operator when the patient work of breathing is within the desired work of breathing.
The above and other objects and advantages of the present invention will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings.